How do I finance and build correctly? Solid house or prefabricated house?

Well, if the property corresponds to the value of the investment, the OP has already contributed €80k in equity. If he is satisfied with a small house, that should be possible. If he can also provide personal contributions or involve acquaintances, all the better.

Honestly, it annoys me and it doesn't help the OP either.

Yeah, annoying, right

The best help for the thread starter is the advice to save new equity. Nothing more should be done until the plans become more concrete. The individual income is very low compared to an average family building a house. I don't think it can be managed by him alone.

I think that one, and especially individually the OP, can best assess this when a few figures of a possible financing are presented. I consider it inappropriate to make any generalized statement now since there is overall far too little information available. Especially regarding the project concerning the size of the building, the personal expectations regarding the equipment, and the type of house. Questions that arise for me in advance: 1) In which city is the plot located? 2) Marital status? Will a partner possibly move into the house? 3) How large should the house be in square meters? 4) Are there upscale demands regarding the equipment of the house? 4) Currently no equity available at all? 5) Could the plot be divided, respectively a part separated and sold so that equity is available?

I told you concrete plans are missing
But it is also difficult when the thread starter actually wants to live his life independently, but now should decide on a number of children in order not to build the house too small from the start. On the other hand, he (presumably) does not yet live together with his partner.

I add my recommendation for a rental apartment for the next few years. The plot can still be taken anyway.

At least it would be important to have a cost figure as a basis or alternatively a desired interest and repayment rate from which the cost figure can be calculated.

Also important is the knowledge about the investment location due to the inclusion of regional banks.

Let's just assume that it is not an economically strong region / rather rural - otherwise the parents would not offer 1,600 sqm for €80,000 apartment renovation. Then it will be a small standard house = €250,000 plus own work. Thus, approximately €250,000 needs to be financed with reasonable equity. Relatively young client, so 2% repayment with rather longer financing term. That corresponds to a comfortable 4% annuity of €833 monthly and is therefore well affordable with his income (assuming no existing liabilities). This gives the client a first, realistic assessment. If things get serious with the girlfriend and children are planned, he can also build bigger if the girlfriend is jointly liable.